Intramolecular charge transfer enables highly-efficient X-ray luminescence in cluster scintillators

Luminescence clusters composed of organic ligands and metals have gained significant interests as scintillators owing to their great potential in high X-ray absorption, customizable radioluminescence, and solution processability at low temperatures. However, X-ray luminescence efficiency in clusters is primarily governed by the competition between radiative states from organic ligands and nonradiative cluster-centered charge transfer. Here we report that a class of Cu4I4 cubes exhibit highly emissive radioluminescence in response to X-ray irradiation through functionalizing biphosphine ligands with acridine. Mechanistic studies show that these clusters can efficiently absorb radiation ionization to generate electron-hole pairs and transfer them to ligands during thermalization for efficient radioluminescence through precise control over intramolecular charge transfer. Our experimental results indicate that copper/iodine-to-ligand and intraligand charge transfer states are predominant in radiative processes. We demonstrate that photoluminescence and electroluminescence quantum efficiencies of the clusters reach 95% and 25.6%, with the assistance of external triplet-to-singlet conversion by a thermally activated delayed fluorescence matrix. We further show the utility of the Cu4I4 scintillators in achieving a lowest X-ray detection limit of 77 nGy s−1 and a high X-ray imaging resolution of 12 line pairs per millimeter. Our study offers insights into universal luminescent mechanism and ligand engineering of cluster scintillators.

added dropwise under stir and further reacted for 16 h. Then, a solution of chlorodiphenylphosphine (7.4 mL, 40.7 mmol) in diethyl ether (10 mL) was added dropwise. The cold bath was removed, and the reaction mixture was stirred for another 16 h. After then, the reaction was quenched with water (50 mL). The mixture was extracted with dichloromethane (3 × 50 mL). The organic layer was combined and dried with anhydrous Na 2 SO 4 . The solvent was removed in vacuo. The residue was purified by flash column chromatography with the eluant of DCM:PE (1:10) to afford the ligand as white powder of 7.09 g with a yield of 80%. 1  were dissolved in 20 mL of CH 2 Cl 2 . The mixture was stirred for 4 h. Then, the solvent was evaporated to obtain crude complex, which was further recrystallization from CH 2 Cl 2 /ether solution to afford 1.3 g of white crystal with a yield of 82%. 1   All diffraction data were collected at 295 K on a Rigaku Xcalibur E diffractometer with graphite monochromatized Mo Kα (λ = 0.71073 Å) radiation in ω scan mode. All structures were solved by direct method and difference Fourier syntheses. Non-hydrogen atoms were refined by full-matrix least-squares techniques on F2 with anisotropic thermal parameters. The hydrogen atoms attached to carbons were placed in calculated positions with C−H = 0.93 Å and U(H) = 1.2Ueq(C) in the riding model approximation. All calculations were carried out with the SHELXL97 program. Absorption and photoluminescence (PL) emission spectra of the target compound were measured using a SHIMADZU UV-3150 spectrophotometer and a SHIMADZU RF-5301PC spectrophotometer, respectively. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on Shimadzu DSC-60A and DTG-60A thermal analyzers under nitrogen atmosphere at a heating rate of 10 °C min -1 . Cyclic voltammetric (CV) studies were conducted using an Eco Chemie B. V. AUTOLAB potentiostat in a typical three-electrode cell with a glassy carbon working electrode, a platinum wire counter electrode, and a silver/silver chloride (Ag/AgCl) reference electrode.

III. Theoretical Simulation
Density functional theory (DFT) and time-dependent DFT (TDDFT) computations were carried out with different parameters for structure optimizations and vibration analyses. The ground state (S 0 ) configuration was established according to single crystal data. The S 0 , singlet and triplet states in vacuum were simulated by the restricted and unrestricted formalism of Beck's three-parameter hybrid exchange functional 2 and Lee, and Yang and Parr correlation functional 3 B3LYP/6-31G(d,p) for ligands and double-ζ LANL2DZ basis sets for CuI, respectively. The fully optimized stationary points were further characterized by harmonic vibrational frequency analysis to ensure that real local minima had been found without imaginary vibrational frequency.
The total energies were also corrected by zero-point energy both for the ground state and triplet state.
Natural transition orbital (NTO) analysis was performed on the basis of optimized ground-state geometries at the same level. 4 The contours were visualized with Gaussview 5.0. All computations were performed using the Gaussian 09 package. 5  plug-in card for data processing. A temperature controller was equipped to achieve 11-500 K variation.

Photoluminescence quantum yield measurement
Photoluminescence quantum yields (PLQY, ϕ PL ) of these films were measured through a Labsphere 1-M-2 (ϕ = 6'') integrating sphere coated with Benflect having efficient light reflection from 200-1600 nm, which was integrated with FPLS 1000. The absolute ϕ PL determination of the sample was performed with two spectral shows temperature-independent excitation spectra, owing to suppressed nonradiation at high temperature.
Excitation spectra of ligand-centered charge transfer emissions from [DBFDP] 2 Cu 4 I 4 also keep stable, but its 3 CC attributed excitation spectra indicate that two cluster-centered bands are weakened at high temperature, due to triplet quenching. At higher temperature, the identical excitation spectra of both ligand-and cluster-centered emissions demonstrate the enhanced energy transfer from high-energy MLCT/ILCT/LCT to MICT and also the superiority of the former in quenching suppression. V. X-Ray luminescence Analysis 1. Room-temperature radioluminescence measurement 100 mg of the sample was pressed into 7 mm slices, and then gently placed into the sample tank to complete the sample loading. The voltage and current parameters to be tested are entered from the control panel and the radiation source was switched on. At the same time the spectrum scan was started. The spectrometer and radiation source were Edinburgh FS5 and Amptek Mini X-ray (4W), respectively. [DDPACDBFDP] 2 Cu 4 I 4 at 50 kV under in the current range of 10-80 μA.

Temperature-dependent radioluminescence measurement
The 10 mg sample was spread gently onto the testing dish to cover the bottom. The test-bench temperature and embedded in water for one month and heated for ten minutes. After exposed to UV (365 nm) or water for one month or heated at 250 o C for ten minutes, luminance reduction of [DDPACDBFDP] 2 Cu 4 I 4 can be limited within 10% of initial intensities, indicating the outstanding stabilities under harsh conditions ( Supplementary   Fig. 15). The high photo-and thermo-stabilities of the cluster benefit the improvement of device duration for their CLEDs in electroluminescence process, since it is believed that device aging is mainly induced by photo-and thermo-degradation of the materials. (%) [DDPACDBFDP] 2 Cu 4 I 4 230, 287, 322 [a] 252, 302, 391 [b] 510 [b]

Device fabrication
Before loading into a deposition chamber, the ITO substrate was cleaned with detergent and deionized water, dried in an oven at 120 °C for 4 h, and treated with oxygen plasma for 3 min.

EL measurement
EL spectra and CIE coordinates were measured using a PR655 spectrum colorimeter.
Current-density-voltage and brightness-voltage curves of the devices were measured using a Keithley 4200 source meter and a calibrated silicon photodiode. All measurements were carried out at room temperature under ambient conditions. For each structure, four devices were fabricated in parallel to confirm performance repeatability. The data reported herein were those closest to the average results.  Table S2).
In contrast, CzAcSF:[DBFDP] 2 Cu 4 I 4 showed single-band sky-blue electroluminescence emission ( Supplementary Fig. 19). It means excitons were firstly formed on CzAcSF matrix, which then converted triplets to singlets, therefore limited triplet capture by MICT state.